Adoption of electronic health records (EHRs) has been increasing rapidly in recent years, driven by an unprecedented investment from the U.S. government. EHRs hold the potential to facilitate dramatic improvements in patient care but they are only tools-they do not, in and of themselves, alter states of disease or health. As such, realizing the anticipated quality and efficiency gains from EHRs largely depend on how they are used. Practice-- and individual--level decisions about how to capture data within EHRs impacts the ability of EHR data to support key activities -- like care coordination and population health management -- that will result in performance gains. Although clinicians need flexibility to adapt EHR use to different clinical scenarios, variation in EHR documentation could result in a disorganized record and encumber the delivery of efficient and high- quality care. There is limited prior research that captures the extent of variation in who uses the EHR (e.g., physician, medical assistant, or scribe), what they are capturing in the EHR (e.g., what information is recorded), and how it is captured (e.g., structured templates or free-text). We also understand little about when such variation may be problematic or what strategies may help. The proposed dissertation uses mixed quantitative and qualitative methods to create a robust understanding of variation in EHR use in outpatient, primary care practices. First, multi-level modeling of task- lo data from a national, cloud-based commercial EHR vendor will serve to quantify variation in EHR documentation in order to identify the most prevalent forms of variation and the root source(s). Second, interviews with clinicians and staff in primary care practices will identify potentially problematic forms of variation in EHR documentation that inhibit the ability of EHRs to improve care. Interviews will also identify organizational strategies to address problematic variation in EHR documentation in order to develop specific recommendations for optimizing EHR use. This multi-faceted research will enhance efforts by physician practices, EHR vendors, and policy makers to ensure that EHRs facilitate improvements in care delivery by focusing on the phenomenon of variation in EHR documentation and providing rich descriptions of common problems and potential solutions. The proposed dissertation will support AHRQ's interest in effectively using health information technology for quality improvements by contributing to our understanding of how EHRs are used once they are implemented (Research Area of Interest 3), and our understanding of how variation in EHR documentation moderates the relationship between EHR implementation and important health care system outcomes (Research Area of Interest 4), as expressed in the Special Emphasis Notice NOT-HS-13-011.